It is well known that there is a difference between true solar time, which is the time that elapses between two consecutive meridian passages of the sun at the same location, and mean solar time or civil time which is the mean duration in a year of all the true solar days. This difference between civil time and true solar time reaches +14 minutes 22 seconds on 11 February and −16 minutes 23 seconds on 4 November. These values vary very little from year to year.
To indicate the time difference between civil time and true time, in addition to the hand that indicates the minutes of civil time, some timepieces include a so-called equation of time mechanism which includes a hand that moves opposite a graduated scale to indicate the difference between the minutes of civil time and the minutes of true solar time on a given day. This true solar minute hand is actuated by an equation of time cam whose profile is determined by the difference between the mean solar time and the true solar time on every day of the year.
Another mechanism for indicating the time difference between civil time and true time is known by the name of a ‘running equation of time’. The hand arrangement of a timepiece equipped with a running equation of time mechanism comprises two concentric minute hands, one indicating the minutes of civil time and the other indicating the minutes of true time. At any moment, the distance between the civil minute hand and the true solar minute hand is determined by the difference between the mean solar time and the true solar time on the day of the year concerned. Like the equation of time mechanism, the true solar minute hand of a running equation of time mechanism is actuated by an equation of time cam.
The equation of time cam is driven in rotation at the rate of one revolution per year using either a simple or perpetual calendar mechanism. The simple calendar mechanism is arranged to indicate the day of the week, the day of the month, the month of the year or the phases of the moon, but does not take account of the variation in the number of days in the months (months of 28, 29 or 30 days). In other words, the user of a watch with a simple calendar mechanism will have to make a manual correction at the end of every month with less than 31 days. For example, on 28 February or 30 April, a manual correction will have to be made. The perpetual calendar mechanism, like the simple calendar mechanism, can indicate the day, the date, the month and the phases of the moon. However, unlike a simple calendar mechanism, a perpetual calendar mechanism automatically takes account of the length of the months (28, 29 and 30 days) without manual intervention. A perpetual calendar mechanism thus automatically takes account of leap years.
An example of a running equation of time mechanism is disclosed by EP Patent Application Publication No 1286233A1 in the name of the Applicant. FIG. 1 annexed to this Patent Application is taken from the aforementioned EP Patent Application Publication No 1286233A1 and illustrates a running equation of time mechanism driven by a differential device.
This Figure also shows an equation of time cam 1, whose profile is determined by the difference, on every day of the year, between the mean solar time or civil time and true solar time. This equation of time cam 1 is driven in rotation at the rate of one revolution per year using a simple or perpetual calendar mechanism contained in the timepiece. Equation of time cam 1 carries a month disc 2 which rotates at the same speed as cam 1 and which makes the position of equation of time cam 1 coincide with the date indicated by the calendar mechanism, so that the solar time minute hand 4 indicates the exact difference between the minutes of civil time and the minutes of true solar time.
The simple or perpetual calendar mechanism may be of any known type and will not be described in its entirety here. To ensure proper understanding, it is sufficient to know that this calendar mechanism drives equation of time cam 1 at the rate of one complete revolution per year. However, for the purpose of illustration only, a date wheel set 6 driving a hand 8 which indicates the date (from 1 to 31) is represented. This date wheel set 6 rotates at the rate of one complete revolution per month. It is actuated by the calendar mechanism and drives equation of time cam 1 via an intermediate date wheel 10 which can reverse the direction of rotation, and a reduction wheel set 12 which can reduce the rotational speed from one complete revolution per month to one complete revolution per year.
The solar minute hand 4 is driven by a differential gear device 14 which has as respective inputs a gear train driving a civil minute hand 18 and a rack 20 which cooperates with equation of time cam 1 (rack 20 is represented in FIG. 1 in both of its end positions, once in a solid line and the other time in dot and dash lines). More specifically, as seen in FIG. 1, differential gear device 14 includes at least one and preferably two planetary pinions 22 driven by the motion work of the watch movement. These two planetary pinions 22 are capable of rotating on themselves and rolling over the inner toothing 24 of an equation of time wheel 26. The latter also has, on the external periphery thereof, a first toothed sector 28 via which it cooperates with a second toothed sector 30 arranged on one of the ends of rack 20. This rack 20 is subjected to the return action of a spring (not represented) which is fixed to the watch frame and which tends to place a feeler spindle 32, forming the other end of rack 20, against the profile of equation of time cam 1. The solar time display gear train includes a solar time display pinion 34 placed at the centre of differential gear device 14. This solar time display pinion 34 meshes on one hand with planetary pinions 22, and carries, on the other hand, a solar time display wheel 38 which meshes with a cannon-pinion 40 onto the pipe of which solar minute hand 4 is pressed. This gear train 38, 40 returns the solar minute display to the centre 42 of the watch movement, so that the solar minute hand 4 is concentric with civil minute hand 18.
The running equation of time mechanism which has just been described operates as follows.
In the normal operating mode of the watch, equation of time cam 1, rack 20, and therefore equation of time wheel 26, are immobile. However, planetary pinions 22 are driven by the watch movement. Thus, they rotate on themselves and roll over the inner toothing 24 of equation of time wheel 26, driving solar time display pinion 34 in rotation, which allows solar minute hand 4 to rotate concomitantly with civil minute hand 18. The distance between solar minute hand 4 and civil minute hand 18 thus remains constant over a 24-hour period.
Once per day, at around midnight, equation of time cam 1 pivots, driven by the calendar mechanism which changes the date from one day to the next day. At that precise moment, feeler spindle 32, which is in contact with the profile of equation of time cam 1, in turn pivots rack 20. As it pivots, rack 20 drives equation of time wheel 26 in rotation. Planetary pinions 22, which are substantially immobile during this brief time interval (they make one complete revolution on themselves in one hour), rotate on themselves, driven in rotation by equation of time wheel 26, and in turn drive solar time display pinion 34 so as to precisely set the position of the solar minute hand again.
The running equation of time mechanism described above therefore makes it possible to display, at any time, the time difference between mean solar time and true time, by means of a civil minute hand and a solar minute hand. It is noted, however, that differential gear device 14 is not located at centre 42 of the watch movement. The design is therefore not symmetrical, which is counter-intuitive. Further, owing to the off-centre position of differential gear device 14, it is necessary to provide an additional gear train (solar time display wheel 38 and cannon-pinion 40) to return the solar time display to centre 42 of the watch movement and ensure concentricity between civil minute hand 18 and solar minute hand 4. The additional gear train occupies space and may cause failure.